There are various types of image display devices for displaying a high definition television image on a large screen. PDPs belong to one of the various types. A PDP is hereinafter described as an example.
A PDP is formed of two glass substrates: a front-side glass substrate for displaying an image; and a back-side glass substrate facing the front-side glass substrate. The front-side glass substrate has the following elements:                a display electrode that is formed on one principal surface thereof and includes a stripe-like transparent electrode and a bus electrode;        a dielectric film that covers the display electrode and works as a capacitor; and        a MgO protective layer formed on the dielectric film.        
The back-side glass substrate has the following elements:                a stripe-like address electrode formed on one principal surface thereof;        a dielectric film for covering the address electrode;        barrier ribs formed on the dielectric film; and        phosphor layers that are formed between the barrier ribs and emit red light, green light, and blue light, respectively.        
As the front-side glass substrate and the back-side glass substrate, glass substrates that are easily increased in area, have high flatness, are inexpensive, and are manufactured by a float method are used. These glass substrates are disclosed in Electronic Journal, Separate Volume “2001, FPD Technology Summa” (Electronic Journal Co. Ltd. Oct. 25, 2000, p706-p707).
The float method is a method of forming plate-shaped glass by floating and conveying molten glass onto molten metallic tin under reducing atmosphere. An inexpensive glass sheet having large area can be precisely manufactured in the float method, so that the float method is in widespread use in manufacturing of a window glass or the like.
When an Ag electrode made of silver material is formed on a float glass substrate manufactured by the float method, however, a colored layer is disadvantageously formed on the surface of the glass substrate and the glass substrate changes into yellow (yellows).
This coloring phenomenon of the glass substrate by the Ag electrode is caused by the following processes:                a silver colloid is generated by oxidation-reduction reaction between reducing bivalent tin ions (Sn++) existing on the glass substrate and silver ions (Ag+); and        light absorption therefore occurs near wavelength of 350 to 450 nm.        
In other words, the glass substrate is exposed to the reducing atmosphere containing hydrogen in a molding process in a float furnace as a molten metallic tin bath. A reducing layer with a thickness of several μm containing tin ions (Sn++) of the molten tin (Sn) is generated on the surface of the glass substrate. When a bus electrode including an Ag electrode is formed on the glass substrate having the reducing layer on its surface, silver ions (Ag+) separate from the bus electrode, and infiltrate into the glass due to ion exchange with alkali metal ions contained in the glass. The infiltrating silver ions (Ag+) are reduced by the tin ions (Sn++) existing in the reducing layer to generate metallic silver (Ag) colloid. The metallic silver (Ag) colloid yellows the glass substrate. The yellowing also occurs on the front-side glass substrate having the bus electrode on the transparent electrode.
When the glass substrate, especially the front-side glass substrate, yellows, the yellowing is fatal in the image display device. Due to the yellowing of the glass substrate, the panel looks yellow, the commercial value decreases, display brightness of blue decreases to change display chromaticity, and color temperature decreases to degrade picture quality especially in displaying white.
These problems occur not only in a PDP but also in a general image display device having a structure where an Ag electrode is formed on a glass substrate.
The present invention addresses the problems, provides an image display device allowing good image display by suppressing yellowing of the glass substrate, and provides an evaluating method of the glass substrate for use in the image display device.